The broad objective of this project, since its inception, has been to understand how IgG antibodies in man mediate their biological effects. IgG antibodies bind and cluster Fc receptors (FcgammaR) on cell membranes to initiate several molecular processes that all converge to eliminate antigen. The project strives toward a detailed characterization of these molecular mechanisms. Efforts, concentrating on the molecules involved, especially in leukocytes, have of late yielded a description of a family of FcgammaR that are in part initiators of these mechanisms. How this family of FcgammaR triggers the variety of responses, what the most proximate signals are, what subunits are involved, what is meant by clustering, are all gaps in our knowledge that require concentrated attention. More recently, the project has focussed on the high affinity FcgammaR, has characterized 3 genes for this FcgammaR, has described 6 transcripts, and has shown that the prototype FcgammaRI is noncovalently linked with the gamma-chain of the FcepsilonRI complex. The project will continue its concentration on this one sub-family of FcgammaR, aspiring to key insights into FcgammaR function, cell triggering, and antibody action. The projects intermediate objectives are two-fold, to analyze the structure-function relationships of the prototype FcgammaRI molecule, and to characterize the other 5 recently described FcgammaRI molecules, focussing on the b2 form. The immediate specific aims are five. The project will 1) complete its characterization of the 6 FcgammaRI transcripts already in part described, determining in which cells these transcripts are found; 2) characterize the proteins translated from these transcripts, concentrating on the b2 form which is an apparent integral membrane protein missing the unique third extracellular domain; 3) assess these proteins for their capacity to mediate a variety of functions characteristic of FcgammaR such as recycling, endocytosis, and [Ca++]i flux; 4) study the structural details and the functional implications of the association of FcgammaRI with the gamma-chain; and 5) map the 3 FcgammaRI genes in greater detail to their locus on chromosome 1. Distinct opportunities for progress on each of these 5 specific fronts is apparent from on-going experiments. Although the thrust of the effort is on basic molecular mechanisms, the biological processes under scrutiny are crucial for antibody activity. Thus, the project's findings will find wide application to health and disease in a variety of areas, including infection, cancer, and autoimmunity. Already, antibodies or FcgammaR which this project helped describe are being exploited for cytotoxic purposes and appear central to the pathophysiology of some autoimmune diseases. The research design employs the in vitro methods of cell biology, molecular biology, biochemistry, and immunology to examine the selected mechanisms at the molecular level.